High-pressure discharge lamps, as are used, for example, as video projection lamps, generally have two identical electrodes, which are usually rod-shaped. During operation of such high-pressure discharge lamps with alternating current, very disruptive flicker phenomena may arise. These flicker phenomena come about as a result of varying jumping of the root point of the arc at the electrode tips. This is made possible by the frequent change in the electrode function from the anodic (positive polarity) to the cathodic phase (negative polarity) at the operating frequency. Such jumping of the arc root in particular impairs the application of high-pressure discharge lamps in optical devices, for example projection devices, video projectors, microscope lights and can even result in unusability in the application.
U.S. Pat. No. 5,608,294 has disclosed, for low-frequency (50 Hz up to a few 100 Hz) operation of a high-pressure discharge lamp, superimposing short synchronous pulses on the square-wave lamp current profile for stabilization purposes, i.e. for preventing jumping of the root point of the arc. In this case, the current is increased for a short period of time at the end of a half cycle prior to subsequent commutation. In accordance with the cited document, the current pulse prior to commutation results in a short-term temperature increase at the live root points of the arc on the electrodes, primarily the instantaneous anode. As a result, material is deposited, i.e. the electrode metal tungsten from the gas cycle process is deposited with the tungsten halides on the electrodes, and peak formation occurs on the electrodes, which very effectively stabilizes the discharge and the root of the arc.
WO 03/098979 A1 has disclosed the operation of a high-pressure discharge lamp with an unmodulated RF signal of more than 3 MHz. In general, high-pressure discharge lamps permit successful RF operation only above frequencies which are above the acoustic resonances in the combustion chamber. These acoustic resonances result in strong flows in the combustion chamber which generally markedly disrupt the discharge arc. However, the literature mentions approaches for damping the acoustic resonances by means of suitable feed currents or of avoiding them entirely. By way of example, reference is made to DE 10 2005 028 417.5 and DE 10 2005 059 763.7. However, such solutions are usually very involved.
Finally, reference is made to DE 198 29 600 A1, which is concerned with RF operation of a high-pressure discharge lamp. It relates likewise in particular to the problem of jumping of the root of the arc on the electrode tips. It proposes the solution, on the basis of a prior art in which the high-pressure discharge lamps have been operated at a frequency of below 2 kHz, of operating the lamp at a frequency of above 800 kHz, preferably above 1 MHz and particularly preferably between 2 and 3 MHz. In a preferred development, the operating frequency is swept both continuously and suddenly with a modulation frequency of less than 10 kHz, preferably between 1 and 2 kHz. Although this can under certain circumstances for certain high-pressure discharge lamps represent a solution, this measure has proven to be ineffective in the high-pressure discharge lamps investigated by the inventors of the present invention.